1. Field of the Invention
The invention relates generally to systems for controlling the presence of contaminants in chemical reaction systems and, more particularly, the invention relates to methods and apparatus for removing contaminants found in urea hydrolysis reactors and others wherein contaminants are formed in the course of an ongoing reaction.
2. Description of Background Technology
The Problem of Contaminant Formation
Solid urea is available in several commodity grades destined for uses either in agricultural applications as fertilizers or in chemical process for production of urea-formaldehyde resins and thermosetting polymers. Solid urea is available in bags or bulk quantities, and is commonly transported in bulk truck or railcar loads. Urea is a non-hazardous material, and affords a safe starting material in certain process requiring gaseous ammonia as, for example, in processes for removal of nitrogen oxides from the tail gas stream of combustion processes, such as in fuel-fired boiler operations in public electric power generation plants. Such processes use gaseous ammonia in Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR) methods for removal of nitrogen oxides. Another example is the use of gaseous ammonia to treat fly ash in the tail gas system of a fuel-fired boiler in a public electric power generation plant. Such treatment is beneficial for collection of the fly ash. Common alternatives to use of urea include use of anhydrous ammonia or of aqueous ammonia, both of which are hazardous chemicals presenting serious risks in the transport, handling, storage, and use with attendant regulatory compliance requirements.
Urea may be hydrolyzed to form gaseous ammonia for such uses. Such processes typically employ solid urea supplied in bulk quantities, and of a composition readily available as a commodity. Solid urea is a relatively soft solid that is hygroscopic, which may cause problems in handling it in a highly pure form. Therefore, it is a common practice in the industry to add certain chemical compounds to the solid urea to improve its physical properties and to improve handling characteristics of a granulated or prilled urea product. Such additives include but are not limited to those disclosed in Belasco et al. U.S. Pat. No. 3,248,255 (gaseous formaldehyde to form urea-formaldehyde resin surface coating), Van Hijfte et al. U.S. Pat. No. 4,160,782 (dimethylolurea and/or trimethylolurea), Elstrom et al. U.S. Pat. No. 4,204,053 (formaldehyde), Blouin U.S. Pat. No. 4,587,358 (lignosulfonates), Gallant et al. U.S. Pat. No. 5,102,440 (urea-formaldehyde compounds), and Kayaert et al. U.S. Pat. No. 5,653,781 (formaldehyde, methylolureum (methylolurea), urea-formaldehyde pre-condensates, or hexamethylenetetramine), the disclosures of which are incorporated herein by reference.
Such additives are typically present in the solid urea in concentrations up to 2 wt. %. Therefore, during the continuous use of such solid urea in a hydrolysis reactor designed to produce gaseous ammonia, the solid urea is first dissolved into an aqueous solution, commonly in the concentration range 20 wt. % to 78 wt. %, preferably in the range 40 wt. % to 60 wt. %, and while the urea is continuously decomposed to ammonia and carbon dioxide and leaves the reactor in a gaseous form, the additives present in the solid urea accumulate and may undergo various chemical reactions that form, along with unreacted additives, a contaminant mass in the reaction mixture.
Several specific process systems have been described and patented with the intent to generate a gaseous stream of ammonia, carbon dioxide, and water vapor at a temperature and pressure useful for removal of nitrogen oxides or treatment of fly ash as described, or for other process applications. Young (U.S. Pat. No. 5,252,308) describes a process system that performs the hydrolysis reaction using aqueous solutions of urea in the presence of catalyst systems, specifically mixtures of ammonium salts of certain polyprotic mineral acids, such as phosphoric or sulfuric acid. Cooper et al. (U.S. Pat. No. 6,077,491) describe a urea hydrolysis process that does not require a catalyst and that may take a variety of forms in the apparatus. Lagana (U.S. Pat. No. 5,985,224) describes a process that employs steam-stripping in the hydrolysis reactor to promote the reaction. The disclosures of Young (U.S. Pat. No. 5,252,308), Cooper et al. (U.S. Pat. No. 6,077,491) and Lagana (U.S. Pat. No. 5,985,224) are incorporated herein by reference.
These processes do not recognize the potential problems that may arise from the accumulation of a contaminant mass in the reactor, nor do they describe why such a contaminant may arise, nor do they provide means to address this contaminant. It is the intent of the invention to describe some of the reasons why contaminants arise in the reactor vessels of the various process systems, and to describe the means to address these contaminants so that maintenance-free urea hydrolysis operation can be achieved or at least the maintenance-free period of operation extended and contaminants removed without the hazards and inconvenience of shutting the system down and opening the reactor vessel for frequent cleaning.
It is an objective of the invention to provide technology for maintaining the operation of reactors by incorporating means to remove soluble and/or insoluble contaminant matter, either intermittently or continuously, from the reaction system.
According to the invention, a method of controlling the amount of such contaminants in a liquid phase reaction mixture in a reaction vessel includes the steps of withdrawing a portion of the reaction mixture containing such soluble and/or insoluble contaminants from the liquid phase and the reaction vessel, separating at least one contaminant from the remainder of the withdrawn portion of the reaction mixture, and recycling at least a portion of the withdrawn reaction mixture to the liquid phase in the reactor vessel.
The invention also provides apparatus for carrying out a chemical reaction in a liquid phase containing such contaminants, including a reactor vessel, means for withdrawing a portion of the reaction mixture from the reactor vessel, means for separating contaminants from the remainder of the withdrawn portion of the reaction mixture, and means for recycling any desired portion of the remainder of the withdrawn portion of the reaction mixture to the reactor vessel.
While the invention will be described with reference to the exemplary urea hydrolysis reaction system, the invention is applicable generally to reaction systems that produce or accumulate soluble and insoluble contaminants during the course of the reaction.